Don't reupload media when sharing to another room

**Is your feature request related to a problem? Please describe.**
When you share media from one Matrix room to another, Element Android reuploads the media unnecessarily.

**Describe the solution you'd like**
Element Android should reuse the existing `mxc://` URI instead of reuploading.

**Describe alternatives you've considered**
A new "Forward" button could be introduced that works like the forward button in Element Web (which is made specifically for sharing media without reuploading). However, this method might be less intuitive for users:

* Forwarding usually implies showing some metadata that the message was forwarded, which isn't possible yet (see [MSC2730](https://github.com/matrix-org/matrix-doc/pull/2730) and [MSC2723](https://github.com/matrix-org/matrix-doc/pull/2723)).
* The difference between "Forward" and "Share" might not be clear.